全文获取类型
收费全文 | 3816篇 |
免费 | 127篇 |
国内免费 | 77篇 |
专业分类
测绘学 | 150篇 |
大气科学 | 313篇 |
地球物理 | 844篇 |
地质学 | 1285篇 |
海洋学 | 305篇 |
天文学 | 835篇 |
综合类 | 29篇 |
自然地理 | 259篇 |
出版年
2022年 | 27篇 |
2021年 | 52篇 |
2020年 | 62篇 |
2019年 | 50篇 |
2018年 | 104篇 |
2017年 | 102篇 |
2016年 | 139篇 |
2015年 | 86篇 |
2014年 | 107篇 |
2013年 | 211篇 |
2012年 | 127篇 |
2011年 | 143篇 |
2010年 | 163篇 |
2009年 | 163篇 |
2008年 | 169篇 |
2007年 | 153篇 |
2006年 | 133篇 |
2005年 | 128篇 |
2004年 | 144篇 |
2003年 | 138篇 |
2002年 | 167篇 |
2001年 | 163篇 |
2000年 | 114篇 |
1999年 | 78篇 |
1998年 | 67篇 |
1997年 | 56篇 |
1996年 | 54篇 |
1995年 | 59篇 |
1994年 | 54篇 |
1993年 | 45篇 |
1992年 | 29篇 |
1991年 | 37篇 |
1990年 | 41篇 |
1989年 | 27篇 |
1988年 | 26篇 |
1987年 | 23篇 |
1986年 | 28篇 |
1985年 | 44篇 |
1984年 | 48篇 |
1983年 | 40篇 |
1982年 | 37篇 |
1981年 | 36篇 |
1980年 | 26篇 |
1979年 | 26篇 |
1978年 | 28篇 |
1975年 | 24篇 |
1974年 | 29篇 |
1973年 | 24篇 |
1972年 | 34篇 |
1971年 | 22篇 |
排序方式: 共有4020条查询结果,搜索用时 31 毫秒
991.
992.
200 0年 4月 5日~ 7日袭击我国西北地区北部、华北、东北南部、黄淮地区的扬沙或沙尘暴波及到长春市 ,带来了大量的降尘。据长春市西部 4个采样点测定 ,4月 7日伴随降雪的降尘量最高可达 13.0 g/m2 ,市区总降尘量超过 30 0 0t。泥雪样品在室温条件下自然融化后 ,经实验室 48h细菌培养 ,其细菌含量为每毫升 8.9~ 10 .5万个 ,每平方米泥雪的细菌侵入量达 6 .4亿个左右。扬沙或沙尘暴对城市造成了巨大的环境危害 ,而伴随着大量微生物的侵入 ,势必引起城市微生态环境的改变 ,威胁到城市的生态安全。 相似文献
993.
994.
Lopamudra Roy 《Geophysical Prospecting》2001,49(1):159-164
A cross-plot of the shape factors and the structural indices, determined from gravity anomalies over various idealized sources, namely horizontal/vertical lines and vertical ribbons with various strike lengths and depth extents, forms a closed loop. Different segments of this loop, termed the source geometry identification loop (SGIL), correspond to different source geometries. Combined use of the structural index and the shape factor determined from an isolated gravity anomaly reduces the ambiguity in characterizing the source geometry. A simulated example and three field examples, namely a Cuban chromite anomaly, an Indian example over manganese ore and a sulphide ore from Quebec, have been analysed by the proposed method in order to identify their respective source geometries. 相似文献
995.
Caracas, Venezuela, Site Effect Determination with Microtremors 总被引:1,自引:0,他引:1
A.-M. Duval S. Vidal J.-P. Méneroud A. Singer F. De Santis C. Ramos G. Romero R. Rodriguez A. Pernia N. Reyes C. Griman 《Pure and Applied Geophysics》2001,158(12):2513-2523
—?Caracas 1967 earthquake caused heavy damage to multi-story buildings. In 1995, 184 microtremor measurement points were performed over the city. The measurement grid was more or less dense and covered the main part of the alluvial basin as well as surrounding rock basement. For each point, the horizontal record spectrum was divided by the vertical one (H/V ratio). Subsequently, the strongest value (Ao) of this ratio in a given frequency band was kept, as well as the frequency (Fo) where it occurred. Spatial interpolations of Ao and Fo were performed among all points of Palos Grandes district. A map was plotted representing a single surface where Ao is relief and Fo is represented by grey gradation. An alluvion thickness (H) map can be compared with this result. Damaged buildings are located on the same map. Fo decreases until 0.6?Hz when alluvion thickness (H) increases. Fo values fit with frequencies previously predicted from computation and with 1967 earthquake observations. Moreover, interpolation surfaces show that amplification (Ao) of H/V on microtremor is quite low above rock but is high on the south part of the basin. The maximum occurs over the non-urbanized zone. However the other area, where Ao is higher than 5, roughly corresponds to the location of the four collapsed buildings. Furthermore, the estimated natural frequency of these buildings was around Fo. Graphics showing H, Fo and Ao evolution through the basin were composed. Links between all these values are noticeable. Fo is claimed to be very similar to the resonance frequency of soil. As for Ao, it could be a fairly relevant sign of damage seriousness. Microtremor technique is an economic tool and it allows measurement grid as dense as desired. H/V ratio processing followed by interpolation of maximum values provides precise and useful information about expected site effect. 相似文献
996.
997.
998.
The nature and restricted range of Dupal-type Sr, Nd and Pb isotopic compositions of Cretaceous kimberlites, tuffaceous diatremes of kamafugitic affinity and carbonatite complexes which intrude the southwestern São Francisco craton margin in Brazil, indicate that these magmas either interacted extensively with, or were derived from, a light rare earth element (LREE) enriched homogeneous lithospheric mantle source isotopically similar to the “enriched mantle I” (EMI) component. The shallow-derived alkalic rocks contain a greater proportion of this EMI-like component, whereas the lower time-averaged Rb/Sr, Nd/Sm and Pb/U ratios of the kimberlites compared to the other rock types suggest mixing of the EMI-like mantle material with variable amounts of mantle with a high 238U/204Pb (HIMU-like) component. Systematic variations in rock types and geochemistry on a regional scale are believed to be indicative of vertical geochemical heterogeneities which are translated into lateral heterogeneities by different depths of melting. It is proposed that HIMU- and EMI-like signatures in particular, are concentrated in laterally extensive but vertically distinctive portions of the mantle beneath the São Francisco craton. The EMI-type signatures appear to be restricted to shallow-derived volcanism, whereas the HIMU-type signatures may originate from a source that started melting deeper in the mantle. The Nd signatures of the EMI-type volcanics follow the evolution path defined by the NeoProterozoic crustal sequences which overlie and flank the craton margin. This suggests that the source of the EMI-type mantle signatures might be related to the tectono-thermal processes which led to the formation and evolution of such crustal sequences. The isotopic similarity of the sources of the studied rocks and of the high-Ti basalts of the northern Paraná basin to those of some Ocean Island Basalts with Dupal signatures in the South Atlantic (viz. in Walvis Ridge) is ascribed to processes by which continental lithosphere became firstly delaminated, and then contaminated a zone of South Atlantic asthenosphere from which hotspot islands have been erupting. 相似文献
999.
P. Pal Roy 《Geotechnical and Geological Engineering》1995,13(2):79-92
Summary This paper substantiates, through three different case studies, that the breakage assessment of opencast bench blasting can be made scientifically through the analysis of joint set orientations of different exposed benches in association with the corresponding face direction. Joint orientations, dip and strike of bedding planes have a significant influence on rock breakage. They can have decelerating effects on blasting performance, which may be rectified by geological mapping, and subsequently modifying the blast design parameters. 相似文献
1000.
Two calc-silicate xenoliths in the Upper Zone of the Bushveld complex contain mineral assemblages which permit delineation of the metamorphic path followed after incorporation of the xenoliths into the magma. Peak metamorphism in these xenoliths occurred at T=1100–1200°C and P <1.5 kbar. Retrograde metamorphism, probably coinciding with the late magmatic stage, is characterized by the breakdown of akermanite to monticellite and wollastonite at 700°C and the growth of vesuvianite from melilite. The latter implies that water-rich fluids (XCO2 <0.2) were present and probably circulating through the cooling magmatic pile. In contrast, calc-silicate xenoliths within the lower zones of the Bushveld complex, namely in the Marginal and Critical Zones, also contain melilite, monticellite and additional periclase with only rare development of vesuvianite. This suggests that the Upper Zone cumulate pile was much ‘wetter’ in the late-magmatic stage than the earlier-formed Critical and Marginal Zone cumulate piles. 相似文献